Detection and ranging of underground anomalies is important for a variety of applications, such as detecting underground cavities, unmapped infrastructure pipelines, and ores. The magnetic method measures anomalies with respect to the earth magnetic field. This method, however, is sensitive to ferromagnetic bodies only. The acoustic method uses sound waves transmitted through the ground and is sensitive to the contrast in acoustic impedance of the anomaly. The EM method uses radio waves and is therefore sensitive to the difference in the EM impedances of the anomaly and surrounding media.
A common EM technique is the ground penetration radar (GPR) in which the transmission and reception of waves is performed from above the ground. This method, however, has limited penetration ability in a ground with high conductivity. In order to increase the depth of penetration, lower frequencies, which suffer less from attenuation, have to be used. However, low frequencies result in low resolution. Typical attenuation factors could be from 1 dB/m to tens of dB/m.
Another EM technique uses radiation from one borehole to another. This method has the same disadvantage of penetration vs. resolution. However, since the signal travels one way (and not into the ground and back as in the GPR method), the penetration vs. resolution is better tolerated. Another advantage of the borehole measurement compared to the GPR is that transmitting and receiving the signal from underneath the ground can reduce the man-made noise level, thereby achieving better performance.
Both methods can be used to create an image of the ground. The GPR can be moved along a path above the ground in order to generate a synthetic aperture image. A cross-section image can be made between the two boreholes by tomographic reconstruction, as described in U.S. Pat. Nos. 5,185,578 and 4,742,305 to Stolarczyk et al. This method requires scanning the locations of the transmitter and the receiver along their boreholes, respectively. This scan is typically done mechanically and takes a long time to complete.
The present invention is most relevant to the borehole tomography technique. The invention exploits the phenomenon that when an EM pulse is injected into a leaky transmission line placed along a dielectric bulk, an EM shock wave will be radiated inside the dielectric bulk, if the signal velocity inside the line is faster than the phase velocity in the dielectric bulk. This radiation is similar to Cerenkov radiation. See D. Grischkowsky, I. N. Duling, III, J. C. Chen, and C.-C. Chi, “Electromagnetic shock waves from transmission lines”, Physical Review Letters, vol 59, pp. 1663-1666 (1987).